Data compression (or reduction) is a digital signal processing technique for reducing the amount of data to be dealt with without losing essential information in the process. This is essentially done by the removal of redundancy in the data and may involve the discarding of uninteresting parts of the data. Such compression can result in some loss of data accuracy. Data compression that allows the exact reconstruction of the original data is often referred to in the literature as lossless. Data compression that involves some reduction in accuracy is known as lossy. Common examples of data compression are "rounding" and "down sampling"; both methods are usually lossy.
Seismic data acquisition requires a large number of seismic experiments to be conducted in order to obtain a reliable image of the Earth's subsurface. Each experiment involves the generation of a sound wave using an appropriate source and measuring the earth's response by a large number of receivers. A large scale seismic survey thus produces an enormous amount of data which will normally be in digital format, which has to be transmitted, stored and processed. To facilitate the handling of such large volumes of data, data compression can be utilised.
A lossy data compression technique that is routinely used in seismic data acquisition is group forming. This involves the retention and transmission and processing of the sum of neighbouring receivers within fixed-sized groups, instead of the individual measurements.
Group forming is not used primarily for data compression. Group forming suppresses random ambient noise and suppresses waves with low apparent velocities, such as groundroll in land seismics. Thus group forming attenuates the high spatial frequency content of the data. However, the attenuation is performed in a crude way as it only partially suppresses apparently slowly propagating waves and alters the rest of the data. Consequently there is a good reason to omit group forming from the acquisition stage and to record the output of every receiver individually. This then permits the application of more sophisticated methods for reducing random and coherent noise. However the abolition of group forming at the acquisition stage greatly increases the amount of data to be handled downstream.
In IEEE Int. SYM. Circuits & Systems, New Orleans, La., May 1-3, 1990, Vol. 2, 1573-6, A. Spanias, S. Johnson et al. describe several transform based methods for seismic data compression. The methods include the Discrete Fourier Transformation (DFT), the Discrete Cosine Transformation (DCT), the Walsh-Hardamard Transform (WHT), and the Karhunen-Loeve Transform (KLT). However the DCT in the form described in the publication and applied to a sliding frame of N data points can be used for a relative comparison between several different transformations. When applied as data compression method, the sliding frame produces a large amount of redundant data in the transform domain.
It is therefore an object of the present invention to provide a method for compressing seismic data. It is another object of the invention to provide a method for compressing seismic data without using group forming.